40 Pirsson — Petrographic Province of Central Montana. 



cases of the lamprophyric dike rocks the biotites have darker 

 borders, otherwise they are very uniform in all classes alike. 



Hornblende. — This mineral is, comparatively speaking, of 

 limited occurrence. It is found in an alkalic type in Square 

 Butte syenite (pulaskose), and in the trachyandesite (adam el- 

 lose) flow on North Willow creek in the Highwoods, and in 

 some of the porphyries composing the laccoliths in the various 

 mountain groups and in vogesite dikes in the Castle and Little 

 Belt Mountains; but, with these exceptions, when it occurs it 

 is clearly uralitic after augite. In this province augite rules 

 in the vast majority of cases and even in the quartzose rocks 

 (quardofelic types) it appears rather than hornblende. 



Feldspars. — It cannot be said that there is any specially 

 marked evidence of consanguinity to be seen in these miner- 

 als so far as the author is able to detect. They do not present, 

 for instance, any such remarkable features as those seen in 

 the feldspars of the alkalic rocks of south Norway, shown in 

 their greatest development in the phenocrysts of the rhombic 

 porphyries. It is to be noted, however, that on account of the 

 tendency for potash to dominate soda in the magmas, that 

 orthoclase or soda-orthoclase is commonly the chief feldspar. 

 Albite is of rare occurrence, even in the strongly alkalic types 

 free from plagioclase, the one instance which is an exception 

 to this— the porphyry of Lookout Butte* in the Little Rockies 

 — being a notable exception. On the other hand, it is an inter- 

 esting fact that in spite of the strong predominance of potash 

 feldspar in so many occurrences of all kinds, microcline may 

 be said to be absolutely wanting in the province. It is prob- 

 ably due to the comparatively recent and hypabyssal character 

 of these rocks and the fact that they have not been subjected 

 to dynamic pressures. 



Absence of minerals. — The characters of a petrographic 

 province are shown as much by the absence of some minerals 

 as by the presence of others, hi this one it is shown by the 

 rarity or absence of minerals caused by the groups of rare 

 earths — as they have, somewhat infelicitously, been called, — 

 that is minerals marked by the presence of zirconia, thoria, 

 cerium, lanthanum, didymium, columbic oxide, etc., etc. Even 

 titanite is a rather rare mineral and zircon uncommon. Expe- 

 rience would seem to show that it is chiefly magmas rich in 

 soda which these oxides accompany and that the potassic domi- 

 nance in the magmas of the central Montana province tends 

 to exclude them and to produce rocks lacking in the interest- 

 ing minerals they give rise to. 



* Jour, of Geol., vol. iv ; p. 422, 1896. 



